For a hybrid vehicle utilizing an internal combustion engine coupled to a step ratio transmission (e.g. conventional planetary automatic transmission, automated single clutch manual, dual clutch transmission, etc.), with at least one electric machine delivering torque to driven wheels after the transmission, there can be powertrain modes that propel the vehicle using only the electric machine with the transmission input coupling open, allowing the engine to be stationary and off. If the vehicle is being driven in such a mode at non-zero speeds, and a total powertrain output torque request exceeds the propulsion capability of the electric motor(s), the engine may need to be started and connected to the driveline to deliver additional torque. The inventors have herein recognized that adding engine torque to driven wheels should be done smoothly and as quickly as possible to reduce delays between a vehicle operator's input and/or expectations, and the actual vehicle acceleration response. In some cases, the engine may need to be connected to driven wheels through a higher transmission gear ratio to achieve higher torque multiplication, to meet high total powertrain output torque requests. At lower powertrain output torque request, the engine may be connected at a lower transmission gear ratio to reduce engine seed, sound volume, and potential driveline disturbance at connection. At higher vehicle speeds, a higher gear ratio may result in a higher input shaft rotational speed that the engine may need to match to lock the input clutch and transmit the requested torque to driven wheels. Since the engine is starting from zero speed, it may take some time to start combustion and accelerate its inertia up to a desired speed to connect the engine to the transmission input. While the engine is accelerating up to the target speed, it may not transmit any torque to the driven wheels, resulting in a delay between a request for engine torque and the engine transmitting torque to the wheels. If the engine quickly connects to the transmission input, but the transmission is in the wrong gear, an additional time delay may result for the transmission to shift to the appropriate gear ratio to meet the required drivetrain torque after the engine connects. If the vehicle is in neutral with no internal shift elements engaged to connect the transmission input to the output, the transmission must first shift into the appropriate gear ratio before the engine can be connected. This adds additional delay to the total driveline torque response, and additional torque must be taken from the driveline (electric motor, vehicle inertia, etc.) to accelerate the rotating inertia of the internal components of the transmission to lock the desired gear before the engine may be connected. The torque for completing such a shift may reduce an acceleration capability of the vehicle being propelled by the electric machine before the engine has been connected to the driveline.
An optimal vehicle response may be achieved if the transmission is in an optimal gear at a time when the engine connects to the driveline. Such a response may be further improved if the transmission is in an optimal gear prior to the engine being commanded to start and connect to the driveline as there may not be any time or energy lost due to shifting the transmission into an optimal gear for connection during the engine starting process. The inventors have thus herein developed systems and methods to at least partially address the above-mentioned issues. In one example a driveline operating method comprises propelling a vehicle solely via an electric machine while an engine of the vehicle is off and not connected to a transmission, the electric machine positioned in the driveline downstream of the transmission, and engaging one or more gears of the transmission with the engine off, to prepare the driveline for an engine start event to meet a vehicle operator torque request.
In one example of the method, engaging one or more gears further comprises engaging one or more gears with a lowest torque multiplication available that allows an input shaft to the transmission to remain above an engine idle speed while the vehicle is propelled solely via the electric machine. In such an example, the method may further comprise shifting the one or more gears to another gear to meet the vehicle operator torque request prior to connecting the engine to the transmission.
In another example of the method, the method may further comprise predicting vehicle operating conditions that may result in the engine start event. In such an example, vehicle operating conditions that result in the engine start event may include a minimum accelerator pedal position and a current vehicle speed. In some examples, engaging the one or more gears of the transmission may further comprise engaging a target gear comprising an optimal gear for connecting the engine to the transmission at the time of the engine start event.
In another example, engaging one or more gears of the transmission may further comprise additionally engaging a non-target gear, where the transmission comprises a dual clutch transmission. In such an example, the non-target gear may comprise a sequentially lower gear than the target gear, provided that the target gear is not the lowest available gear. In another example the non-target gear may comprise a sequentially higher gear than the target gear, provided that the target gear is not the highest available gear.
An example, the target gear may comprise an optimal gear for connecting the engine to the transmission at the time of the engine start event, and where the non-target gear may correspond to an appropriate gear ratio for a current accelerator pedal position and vehicle speed. Furthermore, engaging the one or more gears in the above-mentioned examples may include operating an electric transmission pump to provide hydraulic fluid to actuate one or more shift elements of the transmission. The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.